1. Field of the Invention
The subject invention is related generally to subsea oil and gas production and is specifically directed to a method and apparatus for flushing fluids through the interstices of subsea umbilicals for removing contaminants therefrom.
2. Discussion of the Prior Art
Typically, subsea oil and gas production operations include a surface control and operating platform for operating and communicating with sea floor production equipment. The operating fluids, control signals and control fluids are transported from the surface platform to the sea floor by a plurality of cables, hoses, control lines and the like usually bundled together and encased in a sheath to form an umbilical.
A typical subsea production control and/or chemical injection umbilical will have hoses of various sizes and pressures, fillers such as, by way of example, polypropylene string fillers, an inner sheath, an armor shield and an outer sheath, such that when the bundle is configured in its final assembly, it is of a circular cross-section and may be manufactured by a continuous extrusion process.
The elements that constitute the core of the umbilical, or the operating components, define an inner bundle of and comprise electrical cables, fiberoptic cables suitable for subsea use, fillers essentially round if preformed fillers, and often hollow, particularly if of any decent size. Frequently, there exists an air gap around the fillers and the hoses and electrical cables. Often a polypropylene slit film filling material is also incorporated, all to make cross-section of the umbilical core as circular as possible. This assembly will then be helically taped to form the core bundle.
The taped core is then fed through an extrusion process or roving process to form an inner sheath which may act merely as protection for the core or, in addition, may serve as bedding for armor wire. The sheathed core is then contrahelically wound with wire or the like to provide an armor layer or layers over the core and to add weight to the assembly to help keep it on the bottom of the sea. In most configurations the armor layers are formed of wire. However, braided polymeric materials have been used and the wire is often coated with a noncorrosive layer to extend the life of the umbilical, or just to expand the operating outer diameter of the wire. The armored core is then fed through an extrusion process or roving process, where the outer sheath is added to encase the entire assembly in a continuous shield to protect the umbilical from the subsea environment.
The inner and outer sheaths of the umbilical form separate barriers. The inner sheath is a barrier which surrounds and is in direct contact with the core and provides a base or bed for the armor. In the case of an unarmored umbilical, the inner sheath basically holds the core in assembly and acts as an outer sheath, as well. The inner sheath may, but is not required to, seal the core of the umbilical.
Umbilicals formed in this manner have gained wide spread acceptance, and have achieved substantial success in the industry. Such umbilicals are used to transfer, among other materials, control fluids to the sea floor. There are several types of fluids used including, but not limited to, various corrosion inhibitors, scale inhibitors, hydrate inhibitors, foam inhibitors and the like. Some of these materials are such that they will readily permeate through the typical hose lining material in hoses in the core. Once this occurs, the permeated fluid is free to contact and interfere with the remaining components of the core, often increasing the deterioration rate of the umbilical.
Heretofore, the core materials have been altered or modified in order to eliminate or minimize the permeation rate and the deleterious effects of such permeation. For example, nylons have a fairly high rate of permeation, while Hytrel.RTM. brand materials have a little less rate of permeation, and these are two commonly-used hose liner materials. It has been found that fluoropolymers have much, much lower permeation rates. In any event, all known materials will permit permeation over time, and since umbilicals may be deployed from five to up to thirty years, over a period of time the interstices of the core will become saturated with the permeated fluids.
While minute or small quantities of such permeated fluids or contaminants do not present danger, the hazards increase as the fluids become more concentrated. The permeation phenomenon is such that even where the pressure inside the hose is lower than the hydrostatic pressure outside the hose the permeant will still pass through the hose an into the interstices of the core. While the rate of permeation is somewhat pressure dependent, it still exists.
Methanol, as an example, will permeate from the lower pressure area into the higher pressure area, regardless of the pressure differential. Such permeation will eventually cause contamination of other control fluids as the permeant permeates through the hose liner, into interstices, back through the elements that make up the control line hoses, and into the control fluids inside the hoses. While under normal circumstances such permeation might not be a problem, there are cases where continued permeation can create a serious risk. Not only will the permeation of the methanol dilute it as a control fluid, the permeation process may disturb the lubrication properties or the corrosion inhibition properties of other fluids in the umbilical. Moreover, if the permeated methanol finds its way back into a surface reservoir, for example, as in a closed system where the control fluid is pumped down to the well and is returned via a return line into a surface reservoir, any methanol in the reservoir on the surface platform can create a fire or an explosion hazard. Also, in some cases, over time the methanol can degrade the quality and physical properties of other plastic materials within the core of the umbilical.
To date, the problem has been addressed by altering the core encapsulating materials. While this may ultimately provide a desirable solution, there remains a need for an effective and inexpensive way to solve the permeation problems of new umbilicals as well as to eliminate or minimize the permeation problems associated with the thousands of umbilicals currently installed and in use.